RF power amplifiers are used in a variety of applications such as base stations for wireless communication systems etc. The signals amplified by the RF power amplifiers often include signals that have a high frequency modulated carrier having frequencies in the 400 megahertz (MHz) to 4 gigahertz (GHz) range. The baseband signal that modulates the carrier is typically at a relatively lower frequency and, depending on the application, can be up to 300 MHz or higher.
RF power amplifiers are designed to provide linear operation without distortion. Input and output impedance matching circuits are used to match RF transistors that may have low input and output impedances (e.g., around 1 ohm or less for high power devices), to the impedance matching networks of an external device, such as a circuit board.
A device package for an RF power amplifier can include a transistor die (e.g., MOSFET (metal-oxide semiconductor field-effect transistor), LDMOS (laterally-diffused metal-oxide semiconductor), HEMT (high electron mobility transistor) along with an input and output impedance matching circuit incorporated therein. The input and output impedance matching circuits typically include LC networks that provide at least a portion of an impedance matching circuit that is configured to match the impedance of the transistor die to a fixed value.
In general, there is a tradeoff between the broadband capability and power efficiency in RF power amplifier systems. One example of this tradeoff relates to impedance matching between an RF device package and the circuit board that receives and electrically connects to the RF device package. The circuit board typically includes an impedance matching network (or networks) with a fixed impedance value that is paired to the impedance matching network (or networks) of the packaged device so as to effectuate optimum power transfer between the two. However, in the context of RF signals, maximum transfer efficiency is only attainable at one frequency. If operation of the RF amplifier at another frequency is desired, the designer must reconfigure the impedance matching networks, which requires multiple devices with different internal matching topologies, or accept a reduced efficiency. Another example relates to tuning circuits that are configured to filter out harmonic components of the fundamental frequency. Highly efficient operation is only attained when the harmonics of the RF signal are filtered out. This can be done using reactive components that are part of the input and output impedance matching circuits described above. However, these reactive components are tuned to a particular frequency and become less effective as the device is operated outside of this frequency. As a result, the harmonics are not completely tuned out and the power efficiency of the device degrades substantially.